1. Field of the Invention
The present invention relates to a packet switching apparatus adapted to determine and control an allowed transmission rate to be informed to a transmitting terminal in a packet switching network in which a band management is done by feedback control, and a method of controlling the allowed transmission rate.
2. Description of the Related Art
As one of the communication systems in which feedback control is done, an available bit rate (ABR) service in an asynchronous transfer mode (ATM) network has been proposed. The ABR service is disclosed in the document "ATM forum Traffic Management Specification Version 4.0 R11" (Shirish S. Sathaye, March 1996). In the ABR service, an allowed cell rate (ACR) being a transmission cell rate at a terminal is controlled by a feedback control in which a resource management (RM) cell is used.
FIG. 4 is a schematic block diagram showing the configuration of an ATM network for an ABR service. The operation of the ABR service as in the document "ATM forum Traffic Management Specification Version 4.0 R11" will be described herebelow with reference to FIG. 4.
In FIG. 4, an ATM network is generally indicated with a reference 401. The ATM network 401 incorporates an ATM switch 402 and has connected thereto a transmitting terminal 403 and receiving terminal 404. For sending data to the receiving terminal 404, the transmitting terminal 403 mixes a forward resource management (FRM) cell in a data cell. Upon reception of the data cell through the ATM switch 404, the receiving terminal 404 will change the FRM cell to a backward resource management (BRM) cell and send it back to the transmitting terminal 403.
When the FRM cell sent from the transmitting terminal 403 and the BRM cell sent back from the receiving terminal 404 are passed through the ATM switch 402, the ATM cell 402 will write into either the FRM or BRM cell or into both an explicit rate (ER) indicative of how much congested the connections from the transmitting terminal 403 to the receiving terminal 404 are currently. The ER is a maximum transmission rate at which such connections routed through the ATM switch 402 are allowable without any congestion and which is currently allowed for the transmitting terminal 403. When the connections through the ATM switch 402 are totally a low load, a higher ACR is allowable for the transmitting terminal 403. Therefore, the ER value calculated by the ATM switch 402 will be a large one. On the contrary, when such connections are a larger load, the ATM switch 402 will calculate a smaller ER value. The transmitting terminal 403 will increase or decrease the ACR based on an ER written in the BRM cell sent from the ATM switch 402 to effect a data communications at a rate within the range of increased or decreased ACR.
As mentioned above, the transmitting terminal 403 decreases the ACR because of a received ER value which is lower. In addition, the ACR is decreased due to an initialization thereof. More particularly, if a length of time for which the transmitting terminal 403 sends no FRM cell exceeds an ACR decrease time factor (ADTF), the ACR will be decreased to an initial cell rate (ICR). As described in the aforementioned document "ATM forum Traffic Management Specification Version 4.0 R11", this initialization is an operation effected by the transmitting terminal 403 controlling itself. One of the FRM cell sending conditions is that one FRM cell is sent for a predetermined number of data cells. Thus, since an extremely small number of FRM cells is sent when the transmitting terminal 403 has no further data to send, the ACR initialization will be effected. The purpose of this ACR initialization is for the transmitting terminal 403 to resume with an ICR, not any ACR, a data transmission of which it has once been in pause, in a predetermined time from the pause, thereby preventing the network from being abruptly applied with a large load.
FIG. 5 is a schematic block diagram of an ATM switch in the ATM network, showing the role of the ATM switch. In the network illustrated in FIG. 5, transmitting terminals 502 equal in peak cell rate (PCR) to each other are connected to an ATM switch 504 to links 503, respectively, each having a linkage capacity which is same as the PCR of the transmitting terminals 501 and 502, they are multiplexed by an ATM switch 504, and connected to an ATM network 505 via another link 503. Of the transmitting terminals, one 501 will always send data at a full ACR as the actual transmission rate while the other 502 will send data initially at the full ACR, then at a rate extremely low (LCR) but at which the ACR is not initialized by the transmitting terminal itself, and at the full ACR again in a predetermined time after that.
FIG. 6 is a time chart showing the operations of the transmitting terminal 502 shown in FIG. 5. FIG. 6 shows values of an ER received by the transmitting terminal 502, and an ACR and an actual transmission rate, respectively, of the terminal 502. First, when the transmitting terminals 501 and 502 are continuously sending data, the ER for no congestion of the ATM switch 504 will take a half of a PCR value (will be referred to simply as "PCR" hereinafter), that is, "PCR/2". Accordingly, the ACR of the transmitting terminals 501 and 502 having received that ER will also be "PCR/2". The transmitting terminals 501 and 502 will send data at an actual transmission rate "PCR/2".
In this condition, the actual transmission rate of the transmitting terminal 502 will be reduced to "LCR" at a time t0. The "LCR" is a very small value at which however the ACR will not be initialized. As the actual transmission rate is thus decreased, the ATM switch 504 will be applied with little load for the data transmission by the transmitting terminal 502. Therefore, the ATM 504 will calculate a new ER. Conventionally, the ER will be equal to "PCR" when calculated by the commonly used ER calculation method stated in "Explicit Rate Indication for Congestion Avoidance; ERICA)" in the aforementioned document "ATM forum Traffic Management Specification Version 4.0 R11". The ATM switch 504 will inform the new ER thus calculated thereby to the transmitting terminals 501 and 502.
It should be noted for FIG. 6 that the ER value of "PCR" is informed to the transmitting terminal 502 at a time t2 a predetermined time after the time t0 at which the actual transmission rate of the transmitting terminal 502 has decreased to "LCR". This is because for a time period from the reduction of the actual transmission rate of the transmitting terminal 502 until the terminal 502 is informed of an ER reflecting the reduced actual transmission rate, a propagation delay is caused by a distance between the transmitting terminal 502 and ATM switch 504.
The transmitting terminal 501 has an actual transmission rate equal to "PCR" since the received new ER will lead to an ACR which takes a value "PCR". On the other hand, the actual transmission rate of the transmitting terminal 502 is very low being "LCR", but the ACR will be maintained at "PCR" because the received ER has a value of "PCR".
Next, assume that a factor having reduced to "LCR" the actual transmission rate of the transmitting terminal 502 has been eliminated at a time t5 as shown in FIG. 6. Thus, the actual transmission rate of the transmitting terminal 502 will be raised up to the full ACR. The ACR is not be initialized but maintained at "PCR", so that the actual transmission rate will abruptly jump up to an ACR value, that is, "PCR". Therefore, both the transmitting terminals 501 and 502 will send data at the actual transmission rate "PCR" after the time t5 and so the amount of data arriving at the ATM switch 504 and ATM network 505 will exceed the capacity (equal to "PCR") of the link 503 connecting the ATM switch 504 and ATM network 505 to each other. Thus, a serious congestion will take place at the ATM switch 504. Then, the ATM switch 504 will reduce the ER value to "PCR/2" and inform it to the transmitting terminals 501 and 502.
Because of the delay of a propagation between the transmitting terminals 501 and 502 and the ATM switch 504 as mentioned above, however, the transmitting terminals 501 and 502 will receive the ER of "PCR/2" and starts decreasing the ACR at a time t7 which is a predetermined time from the time t5 at which the congestion has first taken place. For a time period from the time t5 until t7, an extremely high possibility of cell loss will exist in the ATM switch 504.
As having been described in the foregoing, the conventional ATM network is disadvantageous in that if the actual transmission rate of a certain one of the transmitting terminals connected to the ATM network falls, the ACR has an higher ACR reflecting the reduced actual transmission rate, and if the transmitting terminal in question abruptly raises the actual transmission rate up to the full ACR, the ATM network will suddenly be applied with a large load so that a cell loss will take place with a result that the ATM network will incur a lower efficiency of data transmission.